Assembly how to translate JNE to C Code without ZF flag access

Question

ASM to C Code emulating nearly done.. just trying to solve these second pass problems.

Lets say I got this ASM function

401040  MOV EAX,DWORD PTR [ESP+8]
401044  MOV EDX,DWORD PTR [ESP+4]
401048  PUSH ESI
401049  MOV ESI,ECX
40104B  MOV ECX,EAX
40104D  DEC EAX
40104E  TEST ECX,ECX
401050  JE 401083
401052  PUSH EBX
401053  PUSH EDI
401054  LEA EDI,[EAX+1]
401057  MOV AX,WORD PTR [ESI]
40105A  XOR EBX,EBX
40105C  MOV BL,BYTE PTR [EDX]
40105E  MOV ECX,EAX
401060  AND ECX,FFFF
401066  SHR ECX,8
401069  XOR ECX,EBX
40106B  XOR EBX,EBX
40106D  MOV BH,AL
40106F  MOV AX,WORD PTR [ECX*2+45F81C]
401077  XOR AX,BX
40107A  INC EDX
40107B  DEC EDI
40107C  MOV WORD PTR [ESI],AX
40107F  JNE 401057
401081  POP EDI
401082  POP EBX
401083  POP ESI
401084  RET 8

My program would create the following for it.

int Func_401040() {
    regs.d.eax = *(unsigned int *)(regs.d.esp+0x00000008);
    regs.d.edx = *(unsigned int *)(regs.d.esp+0x00000004);
    regs.d.esp -= 4;
    *(unsigned int *)(regs.d.esp) = regs.d.esi;
    regs.d.esi = regs.d.ecx;
    regs.d.ecx = regs.d.eax;
    regs.d.eax--;
    if(regs.d.ecx == 0)
        goto label_401083;
    regs.d.esp -= 4;
    *(unsigned int *)(regs.d.esp) = regs.d.ebx;
    regs.d.esp -= 4;
    *(unsigned int *)(regs.d.esp) = regs.d.edi;
    regs.d.edi = (regs.d.eax+0x00000001);
    regs.x.ax = *(unsigned short *)(regs.d.esi);
    regs.d.ebx ^= regs.d.ebx;
    regs.h.bl = *(unsigned char *)(regs.d.edx);
    regs.d.ecx = regs.d.eax;
    regs.d.ecx &= 0x0000FFFF;
    regs.d.ecx >>= 0x00000008;
    regs.d.ecx ^= regs.d.ebx;
    regs.d.ebx ^= regs.d.ebx;
    regs.h.bh = regs.h.al;
    regs.x.ax = *(unsigned short *)(regs.d.ecx*0x00000002+0x0045F81C);
    regs.x.ax ^= regs.x.bx;
    regs.d.edx++;
    regs.d.edi--;
    *(unsigned short *)(regs.d.esi) = regs.x.ax;
    JNE 401057
    regs.d.edi = *(unsigned int *)(regs.d.esp);
    regs.d.esp += 4;
    regs.d.ebx = *(unsigned int *)(regs.d.esp);
    regs.d.esp += 4;
    label_401083:
    regs.d.esi = *(unsigned int *)(regs.d.esp);
    regs.d.esp += 4;
    return 0x8;
}

Since JNE 401057 doesn't use the CMP or TEST

How do I fix that use this in C code?

Answer 1

The most recent instruction that modified flags is the dec , which sets ZF when its operand hits 0. So the jne is about equivalent to if (regs.d.edi != 0) goto label_401057; .

BTW: ret 8 isn't equivalent to return 8 . The ret instruction's operand is the number of bytes to add to ESP when returning. (It's commonly used to clean up the stack.) It'd be kinda like

return eax;
regs.d.esp += 8;

except that semi-obviously, this won't work in C -- the return makes any code after it unreachable.

This is actually a part of the calling convention -- [ESP+4] and [ESP+8] are arguments passed to the function, and the ret is cleaning those up. This isn't the usual C calling convention; it looks more like fastcall or thiscall, considering the function expects a value in ECX.